If urinary organs are to be observed by an endoscope, an observation space is filled with urine, for example, and a relatively transparent visual field is ensured by causing a perfusate such as a saline to perfuse. The perfusate and urine are both composed mainly of water, and concentrations of a salt, a sugar and the like are not so high and thus, refractive indexes can be considered to be equal to that of water. Thus, in endoscopes intended for observation of these organs, optical specification/performance in underwater observation regarding an object-side medium as water determines practicability.
A point requiring an utmost attention in the endoscope objective optical system for underwater observation is angle narrowing of an underwater view angle with respect to that in an air. A d-line refractive index of water is 1.333 at a normal temperature, and a relationship between the view angle in the air and the underwater view angle when an outer surface of the endoscope objective optical system is a plane is illustrated below:
View angle in the air180°  160°  140°  120°  Underwater view angle97.2°95.3°89.7°81.0°
Using an endoscope for bladder as an example, even if a view angle in the air is relatively wide at 120°, an underwater view angle in actual observation in the bladder is 81°, and a view field range is considerably narrower in the water. In order to search a lesion over the whole region of a bladder inner surface, an operator performs an operation combining operations of curving of a tip end of the endoscope, insertion/removal of an insertion portion, and twisting of the insertion portion, but if the underwater view angle is small, frequencies of these operations are increased, which is not desirable from a view point of work efficiency. By widening the underwater view angle, a burden on an endoscopic operation of the operator can be reduced, and improvement of efficiency of diagnosis/treatment can be expected. Thus, as an objective optical system relating to underwater observation or wide-angle observation of an endoscope, for example, the objective optical system with the view angle in the air at 138.3° at the maximum is disclosed in PTL 1, the objective optical system with the view angle in the air at 180° or more is disclosed in PTL 2, and the objective optical system with the view angle in the air at 190° to 227° is disclosed in PTL 3, respectively.